Repair of DNA strand-breaks and the reconstruction of chromosomal architecture in X-irradiated, proliferative cells will be followed as a function of cell cycle position. The doses used will be in the truly biologically range, viz. 0-1000 rads. The principal analytical methods used will be the alkaline sucrose gradient procedures developed in this laboratory for investigating rapidly-sedimenting DNA species. Two cell types will be employed in the investigation: the Chinese hamster ovary (CHO) line and the very radiosensitive murine leukemic lymphoblast, L5178Y S/S. Attempts will be made with both cell systems to identify the localization of the chromosomal control of the DNA repair mechanisms; and with L5178Y cells the responses of the repair mechanisms will be compared under in vivo and in vitro conditions.